Method and apparatus for electrically adjusting delay in radio-frequency systems

ABSTRACT

An apparatus and method for providing an electrically adjustable RF delay in which a splitter splits an input signal into two signal paths, one signal path providing a delay fixed at an integral number of wavelengths of a desired center frequency and both signal paths providing electrically adjustable attenuation. A combiner combines the signals passing through the signal paths, such that the sum of the electrically-adjustable attenuation provided by the signal paths adds to unity, whereby the input signal is delayed by an adjustable time depending upon the attenuations provided by the signal paths.

This application is a U.S. National Phase Patent Application that claimsthe benefit under 35 U.S.C. § 365 of International Application No.PCT/US2004/031028, filed Sep. 21, 2004, which claims priority to U.S.Provisional Application No. 60/504,684, filed Sep. 22, 2003, the entirecontents of each of which are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to adjustable radio-frequency delayelements. More specifically, the present invention relates to apparatusfor electrically adjusting delay in a radio-frequency systems.

BACKGROUND OF THE INVENTION

Various radio-frequency systems, notably feed forward amplifiers andpredistorters, require the adjustment of delay through signal paths.This is typically done by varying lengths of transmission line, tuningfilters for group delay, adding fixed delay increments, or othermethods. All of these approaches require mechanical adjustment, circuitmodifications, or other operations that greatly complicate themanufacturing process and increase cost. There is a need for a method toelectrically adjust the delay in these applications without introducingcomplications or impairments that render the approach unworkable.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided anelectrically adjustable RF delay element, including a splitter to splitan input signal into two signal paths. One signal path is delayed by afixed integral number of wavelengths of a desired center frequency andboth signal paths are attenuated using electrically adjustableattenuation such that the sum of the electrically adjustable attenuationprovided by the signal paths adds to unity. A combiner then combines thesignals passing through the signal paths. The input signal is delayed byan adjustable time depending upon the attenuations provided by thesignal paths.

Preferably, a balanced variable attenuator provides the splitter andsignal paths and an RF delay element having a delay fixed at an integralnumber of wavelengths of a desired center frequency. The fixed delayelement is connected between port 3 of the balanced variable attenuatorand a first input of the combiner and a second input of the combiner isconnected to port 2 of the balanced variable attenuator, whereby an RFsignal applied to port 1 of the balanced variable attenuator is delayedby an adjustable time depending upon the degree of attenuation providedby the balanced variable attenuator and is provided at the output of thecombiner.

Preferably, the balanced variable attenuator is formed from identicalshunt-mounted reflective attenuators are connected between a pair ofquadrature hybrid couplers or by identical series-mounted reflectiveattenuators are connected between a pair of quadrature hybrid couplers.Preferably, the reflective attenuators are PIN diodes.

According to another aspect of the invention, there is provided a methodfor providing an electrically adjustable delay of an RF signal. Themethod includes the steps of splitting the RF signal into twointermediate signals; delaying one intermediate signal by a fixedintegral number of wavelengths of a desired center frequency;attenuating both intermediate signals by electrically-adjustableattenuation factors such that the sum of the attenuation factors isunity; and combining the intermediate signals into an output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the attached Figures, wherein:

FIG. 1 is a block diagram of an electrically adjustable delay element inaccordance with an embodiment of the present invention;

FIG. 2 is a block diagram of a conventional balanced attenuator usingPIN diodes in a series configuration;

FIG. 3 is a block diagram of a conventional balanced attenuator usingPIN diodes in a shunt configuration;

FIG. 4 is a block diagram of an electrically adjustable delay element inaccordance with an embodiment of the present invention using PIN diodesin a series configuration;

FIG. 5 is a block diagram of an electrically adjustable delay element inaccordance with an embodiment of the present invention using PIN diodesin a shunt configuration;

DETAILED DESCRIPTION OF THE INVENTION

Delay is equal to the negative of the first derivative of phase withrespect to time. As such, it should be realized that any approach usedto adjust delay must not just adjust phase, but actual delay.

In general, each embodiment of the invention uses two signal paths, onethat has a fixed delay greater than the other by N/F₀ where N is aninteger and F₀ is the center frequency of the intended system. Thenon-delayed path is multiplied by a scalar A that can have a value from0 to 1, and the delayed path is multiplied by a scalar equal to (1−A).The two products are then summed, and since the delay difference betweenthem is equal to N wavelengths, they add in phase. The resulting delayof the combined signals is equal to zero when A=1, and is equal to N/F₀when A=0. Between A=0 and A=1, the delay scales linearly. The usefulbandwidth over which this relationship holds true is inverselyproportional to N. In practical application, the limit of N is about 4for reasonable bandwidth.

Referring now specifically to FIG. 1, an electrically adjustable RFdelay element in accordance with an embodiment of the invention isindicated generally by reference numeral 10. The delay element 10 has anRF input port 12 and an RF output port 14. Two signal paths run from theRF input port 12 to the RF output port 14. An input signal received atthe RF input port 12 is split by a splitter 16 into two paths which arerecombined at a combiner 18 and feed to the RF output port 14. The firstof the two paths includes a first attenuator 20 and the second pathincludes a second attenuator 22 and a fixed delay element 24. Asmentioned above the two paths have delays that differ by N/F₀ secondsand the attenuators 20, 22 are adjustable so that the sum of theirattenuations factors is constant.

A difficulty arises in attempting to multiply the delayed andnon-delayed signals by A and (1−A). While it is possible to split aninput signal, multiply the output signals with separate variableattenuators by A and (1−A), delay the second signal, and then recombine,there are problems with this approach. First, A and (1−A) are linearparameters which are difficult to accurately produce using conventionalattenuators, which are typically linear in dB. It is doubly difficult toproduce A and a perfectly matching (1−A). Second, splitting the inputsignal in two and then later recombining yields a minimum insertion lossof 6 dB. It is desirable to keep the insertion loss as low as possible.Hence, while it is believed that an embodiment of the invention shown inFIG. 1 using conventional attenuators can provide an electricallyvariable delay, the other embodiments described below in relation toFIGS. 4 and 5, which utilize modifications of the conventional balancedvariable attenuators depicted in FIGS. 2 and 3, are preferable.

FIG. 2 depicts a conventional balanced variable attenuator, generallyindicated by reference numeral 30, in which identical series-mountedreflective attenuators 32, 34 are connected between an appropriatelyconnected pair of quadrature hybrid couplers 36, 38. Reflectiveattenuators 32, 34 are typically PIN diodes. The conventional balancedvariable attenuator 30 has four ports, usually referred to as ports 1 to4. Ports 1 and 3 are connected to one quadrature hybrid coupler 36,whereas ports 2 and 4 are connected to the other hybrid coupler 38. Port1 is connected to an RF input 40, while port 2 is connected to an RFoutput 42. The port 3 and 4 are terminated with terminators 44, 46. IfPIN diodes are used as attenuators 32, 34, then the attenuation betweenthe RF input 40 and the RF output 42 may be controlled by varying thebias on the attenuators 32, 34 in a manner understood by those skilledin the art.

Similarly, FIG. 3 depicts an alternative form of conventional balancedvariable attenuator, generally indicated by reference numeral 50, inwhich identical shunt-mounted reflective attenuators 52, 54 areconnected between an appropriately connected pair of quadrature hybridcouplers 56, 58. Reflective attenuators 52, 54 are typically PIN diodes.The conventional balanced variable attenuator 50 has four ports, usuallyreferred to as ports 1 to 4. Ports 1 and 3 are connected to onequadrature hybrid coupler 56, whereas ports 2 and 4 are connected to theother hybrid coupler 58. Port 1 is connected to an RF input 60, whileport 2 is connected to an RF output 62. The port 3 and 4 are terminatedwith terminators 64, 66. If PIN diodes are used as reflectiveattenuators 52, 54, then the attenuation between the RF input 60 and theRF output 62 may be controlled by varying the bias on the reflectiveattenuators 52, 54 in a manner understood by those skilled in the art.

The embodiments of the invention shown in FIGS. 4 and 5, are each amodification of one of the conventional balanced variable attenuators30, 50 shown in FIGS. 2 and 3. As such, the same reference numerals areused where appropriate.

In the electrically-adjustable RF delay elements indicated generally byreference numeral 70 in FIG. 4 and reference numeral 80 in FIG. 5, thetermination 46, 66 connected to port 3 of the balanced variableattenuator 30, 50 has been replaced by a delay element 72 connectingport 3 of the balanced variable attenuator 30, 50 to an input of acombiner 74. The delay element 72 provides a fixed delay of N/F₀, asdiscussed above, which means that the phase of a signal passing throughthe delay element 72 is unaffected. Port 2 of the balanced attenuator30, 50 is now connected to the other input of the combiner 74 and theoutput of the combiner 74 becomes the RF output 76 of theelectrically-adjustable RF delay element 70, 80.

The delay provided by either form of electrically-adjustable RF delayelement 70, 80 is varied by adjusting the bias applied to the reflectiveattenuators 32, 34, 52, 54. If the voltage transfer characteristic fromport 1 to port 2 of either form of electrically-adjustable RF delayelement 70, 80 is S21 and the voltage transfer characteristic from port1 to port 3 is S31, then |S31|+|S21|=1. Therefore |S31|=1−|S21|, whichis the characteristic needed to feed the non-delayed and delayed paths.Stated equivalently, if the attenuation from node 1 to node 2 is A, thenthe attenuation from node 1 to node 3 must be (1−A), which is exactlywhat is needed. Further, the phases of the signals at the inputs of thecombiner 74 are the same at −90 degrees. The combiner 74 is shown as anin-phase combiner although it is possible to use a quadrature combineras long as the 90-degree phase difference is accounted for in the delayelement 72.

This approach has two distinct advantages. First, scalars A and (1−A)are accurately created by the action of the modified balancedattenuator. This would be very difficult to accomplish with a pair ofattenuators. Second, the insertion loss of this approach is 3 dB, whichis 3 dB less than the approach using two attenuators.

The above-described embodiments of the invention are intended to beexamples of the present invention and alterations and modifications maybe effected thereto by those of skill in the art without departing fromthe scope of the invention that is defined solely by the claims appendedhereto.

1. An electrically-adjustable RF delay element, comprising a splitter tosplit an input signal into two signal paths, one signal path providing adelay fixed at an integral number of wavelengths of a desired centerfrequency and both signal paths providing electrically-adjustableattenuation, and a combiner having two inputs and an output forcombining signals passing through the signal paths, such that the sum ofthe electrically-adjustable attenuation provided by the signal pathsadds to unity, whereby the input signal is delayed by an adjustable timedepending upon the attenuations provided by the signal paths and isprovided at the output of the combiner.
 2. The electrically-adjustableRF delay element of claim 1, the splitter and signal paths comprising abalanced variable attenuator and an RF delay element having a delayfixed at an integral number of wavelengths of a desired centerfrequency, wherein the fixed delay element is connected between port 3of the balanced variable attenuator and a first input of the combinerand a second input of the combiner is connected to port 2 of thebalanced variable attenuator, whereby an RF signal applied to port 1 ofthe balanced variable attenuator is delayed by an adjustable timedepending upon the degree of attenuation provided by the balancedvariable attenuator and is provided at the output of the combiner. 3.The electrically adjustable RF delay element of claim 2, wherein thebalanced variable attenuator comprises identical shunt-mountedreflective attenuators connected between a pair of quadrature hybridcouplers.
 4. The electrically adjustable RF delay element of claim 3,wherein the shunt-mounted reflective attenuators are PIN diodes.
 5. Theelectrically adjustable RF delay element of claim 2, wherein thebalanced variable attenuator comprises identical series-mountedreflective attenuators connected between a pair of quadrature hybridcouplers.
 6. The electrically adjustable RF delay element of claim 5,wherein the shunt-mounted reflective attenuators are PIN diodes.
 7. Amethod for providing an electrically-adjustable delay of an RF signal,comprising: splitting the RF signal into two intermediate signals;delaying one intermediate signal by a fixed integral number ofwavelengths of a desired center frequency; attenuating both intermediatesignals by electrically adjustable attenuation factors such that the sumof the attenuation factors is unity; and combining the intermediatesignals into an output signal.